Substantial evidence supports an important role for complement in the host defense against neisseria, development of clinical manifestations, potential immunity and morbidity of gonococcal infections. Strains from patients with uncomplicated symptomatic local disease are sensitive to complement mediated killing in normal serum. Patients with disseminated infection are usually asymptomatic at the genital site and harbor strains resistant to complement mediated attack. Both types of isolates activate and fix complement but this reaction occurs slowly on serum resistant strains. More importantly when examined at equivalent degrees of binding to serum sensitive and serum resistant strains, C3 affixed to the latter is functionally bereft. Central to this proposal is the postulate that there are discrete acceptor sites for C3 on the gonococcal surface and that their chemical and physical environment determines whether the C3 will be functionally effective or susceptible to inactivation. Its specific aims are: 1) to characterize and identify C3 acceptor sites on serum sensitive and serum resistant gonococci, 2) to assess the relative affinity of C3b bound to serum sensitive and serum resistant gonococci for factors B and H, and 3) to characterize the basis for cytidine monophosphate-N-acetyl neuraminic acid mediated serum resistance and to initiate studies of this phenomenon at the enzymatic and genetic level. These studies will employ absorbed serum supplemented with affinity purified bactericidal or blocking antibody, intrinsically radiolabelled gonococci, rate limiting quantities of biotinylated C3 and streptavidin columns to harvest gonococcal surface molecules that serve as an acceptor for C3. Kinetic studies will correlate deposition at specific sites with complement function and determine whether bactericidal antibody acts to direct C3 to these sites or to protect C3 from inactivation. Binding of radiolabeled components will be employed to calculate the relative affinity of factors B and H for C3b deposited on gonococci under conditions associated with different outcomes with respect to complement function. Gonococcal and S.minnesota LPS mutants will be utilized to determine the specific structural and physical properties that are responsible for modulating the ability of the membrane attack complex to mediate accelerated decay of the alternative pathway C3 convertase. Bactericidal antibody binding and the susceptibility of C3 to cleavage will be examined on gonococcal strains incorporating sialic acid into their LOS to delineate the basis for their acquired resistance to complement killing. The activity of enzymes predicted to play a role in sialic acid incorporation will be sought and characterized. In total these studies will provide a better understanding of complement related effects important in the host defense against gonococci as well as basic insights relevant to complement biology and microbial pathogens in general.